Characterization of berry ripening in Missouri Norton wine grapes

Abstract

Deciding to harvest hybrid cultivars by sugar accumulation, titratable acidity, orpH likely does not sufficiently model optimal berry ripeness for wine quality, especially for red wine production. The objective of this research was to analytically characterize berry ripening of the interspecific hybrid cultivar, Norton, to assess analytical harvest prediction methods. Representative fruit samples (3 kg) of Norton were collected atweekly intervals from three established vineyards 4 and 7 times prior to harvest in 2015and 2016, respectively, in the weeks between veraison and commercial harvest. In addition to quantifying traditional parameters, organic acids, phenolic compounds, and aroma precursors were measured in both the fruit and the wine produced. Total phenol, tannin and anthocyanin profiles were quantified by Harbertson-Adams’ assay. Important free and total volatile aroma compounds were quantified by GC-MS in wine samples andberry samples incubated with a glycosidase enzyme. Time of harvest within the season,site, and year were each assessed as potential sources of variance, as well as interactionsbetween the three variables. All samples had increasing sugar levels, decreasing titratable acidity, decreasing malic acid and increasing pH over the growing season, although insignificant difference were observed over the three weeks preceding commercial harvest. None of the phenolic parameters showed an overall increase over time, there by not following trends common in Vitis vinifera. Citric acid concentrations negatively correlated with increased wine phenolic concentrations, either indicative of Norton berry metabolism or the influence of citric acid on fermentation conditions. The C-13norisoprenoid β-damascenone concentrations varied by time (0.2-1.5μg/L), and wineconcentrations were successfully predicted by bound concentrations in berry extracts.ixMeasured differences in some volatile compounds were dependent more on site and/or year, including isoamyl acetate and eugenol, whereas others were timing dependent, including β-linalool and methyl salicylate. Although longer time on the vine generally resulted in increased ripeness by traditional parameters, this did not hold true with aroma and phenolic development.